This invention relates in general to vehicle variable assist power steering systems and in particular to a valve for varying the amount of assistance provided by the system in response to a sensed vehicle operating parameter such as vehicle speed.
Power steering systems are included in many vehicles to reduce the effort required by the operator to steer the vehicle. Conventional hydraulic power steering systems include a hydraulic actuator for moving a vehicle steering linkage. The hydraulic actuator is controlled by a rotary control valve assembly which is connected to the vehicle steering wheel. The system provides steering assistance in proportion to steering torque exerted by the operator upon the steering wheel. For any given amount of applied steering torque, the same amount of steering assistance will be provided. However, the amount of assistance needed for low speed maneuvers, such as parking, is not desirable at high speeds, where the assistance detracts from the driver's feel of the road. Thus, it is desirable to vary the amount of steering assist as the vehicle speed varies.
One example of a variable assist power steering system is disclosed in U.S. Pat. No. 4,702,335 issued to Cage et al. In this patent, the supply of hydraulic fluid to the rotary control valve assembly is restricted as vehicle speed increases. The supply is restricted by a flow control valve operated by a stepper motor.
Another example of a variable assist power steering system is described in U.S. Pat. No. 4,561,516 issued to Bishop. The system of this patent is schematically illustrated in prior art FIG. 1. As shown in FIG. 1, the power steering system 10 includes a pump 11 supplying hydraulic fluid through a constant flow valve 12 to a rotary control valve assembly 13. The constant flow valve 12 maintains a constant flow rate for the hydraulic fluid from the pump 11. The control valve assembly 13 supplies fluid to a hydraulic actuator 14 in relation to torque applied to the valve by the vehicle operator. The hydraulic actuator 14 is coupled to a vehicle steering linkage (not shown) and is operable to move the linkage. Fluid is discharged from the rotary valve 13 through a discharge line 15 to a reservoir 16. The hydraulic fluid is drawn from the reservoir 16 through a recirculation line (not shown) to the pump 11.
The power steering system 10 further includes an electronic control module 17 which is responsive to a vehicle speed sensor 18. The control module 17 causes a flapper valve 19 to restrict the flow of fluid through the discharge line 15. Restriction of the fluid flow changes the hydraulic pressure within the rotary control valve assembly 13. In the Bishop patent, changing the hydraulic pressure shifts a component within the rotary valve assembly (not shown) to alter the degree of steering assistance provided to the operator. This relationship is illustrated in FIG. 1 by a block 20 labeled "AXIAL SHIFT" which is connected by dashed lines to the valve 19 and the rotary control valve assembly 13.
A sectional view of the flapper valve 19 is shown in prior art FIG. 2. The valve includes an inlet 21 and an outlet 22. The inlet 21 is connected to the outlet 22 by a chamber 23. A valve element 24 is disposed within the chamber 23 and cooperates with a valve seat 25 to control the flow of fluid through the valve 19. The valve further includes a solenoid coil 26 which urges the valve element 24 towards the valve seat 25 when the coil 26 is energized by an electric current.
During operation of the power steering system 10, a momentum force generated by the flow of hydraulic fluid through the flapper valve 19 urges the valve element 24 to a fully open position. The valve 19 is progressively closed by supplying an increasing electric current to the solenoid coil 26 as vehicle speed changes. As the valve 19 is closed, the flow of hydraulic fluid through the discharge line 15 is restricted, increasing the hydraulic pressure within the rotary control valve assembly 13 and shifting the component. However, the flapper valve 19 includes a response lag between the initial movement of the valve element 24 and a change in vehicle speed. This is because a current must be supplied to the solenoid coil 26 of sufficient magnitude to overcome the fluid momentum force before the valve begins to close.